Incorporating massive antenna arrays into wireless transceivers has been discussed as one of potential techniques to increase link gains via beamforming for specifications of future cellular networks such as the Long Term Evolution (LTE) release 12 and beyond. Possibilities of utilizing massive antenna arrays are under discussions in standardization forums including the third generation partnership project (3GPP). Benefits of using massive multi input multiple output (MIMO) antenna techniques may include power savings, high beamforming gains, low interference generations, robustness of signal transmissions, and so forth. In order to achieve tremendous link gains via beamforming by using massive MIMO antenna techniques with massive antenna arrays, knowledge of very accurate channel state information (CSI) would be required by a transmitter using such technique. If the CSI were to lack accuracy, then beamforming by using lots of antennas would not be brought to its full potential.
However, acquisitions of accurate CSI to be used by a transmitter would often be impractical in various cases, such as frequency-division duplex (FDD) systems for example. On the other hand, reaching high data rates by using spatial domain modulation (SDM) with massive transmit-antenna arrays has recently drawn considerable attentions. Two examples of such SDM schemes may include Spatial Modulation (SM) and Generalized Space Shift Keying (GSSK). In both of these two SDM schemes, CSI may not be needed by transmitters operating under these SDM schemes.
For Spatial Modulation (SM), during each signaling interval, information could be carried by the combination of the antenna space of a transmitting antenna and a transmit symbol of the transmitting antenna. For example, suppose that there are four antennas with each antenna situated at a different location, the space of an antenna could be used to carry information. Since there are four antennas and only one antenna would be turned on per signaling interval, the four antennas could convey four different symbols. For each antenna, additional information could be conveyed by the digital modulation scheme. Assuming that binary phase shift keying (BPSK) is used for all four antennas, two additional symbols could be conveyed by each antenna. Therefore, 8 different symbols could be conveyed by four antennas using the BPSK modulation scheme, and the 8 different symbols could be represented by three bits. Therefore, the number of representable symbols could be increased by either increasing the number of antennas or by using a higher order modulation scheme for each antenna.
For GSSK on the other hand, data could be conveyed by the permutation of different activated antennas as a mapping table could be used to map between input bits and activated antennas.
Accordingly, the SDM schemes have the following advantages. Conveying information in spatial domains would be currently more cost-effective than conveying information in time domains or in frequency domains. By convey information in spatial domains, high data rate could be achieved by a high number of antennas operating under lower-order modulations. For instance, the aforementioned SM scheme would allow 3 bits to be transmitted using 4 transmitting antennas with a BPSK modulation symbol for each antenna. Since only one antenna could be turned on at a time, not only energy consumptions would be lowered but also the hardware cost would be reduced since the hardware of the front end transceivers could be shared by all of the antennas. Furthermore, the CSI is not needed by a transmitter as complex beamforming operations could be accomplished without requiring CSI.
However, there are still several drawbacks in SM and GSSK. If the number of antennas were to stay constant, launching symbols of a higher-order IQ-modulation on activated antennas for the purpose of boosting data rates would be undesirable in noisy channels. For GSSK, the antenna array size and the number of activated antennas have to be increased in order to increase the bit size of a symbol. For instance, in order to represent 64-QAM (6 bits) with 8 antennas, at least 4 antennas should be turned on. To represent 256-QAM (8 bits) with 16 antenna, at least 3 antennas should be turned on. Moreover, maximum-likelihood (ML) detection has been suggested as the receiver's algorithm for both SM and GSSK, the computational complexity to perform ML would be unacceptable in practice despite its optimal performance. These aforementioned challenges could become issues of focus for those who are skilled in the art.